In the distillatory separation of volatile compounds from a compound mixture, chemical engineers are frequently confronted with the case that the compounds possess equal volatility; i.e., form an azeotrope. As a result, these compounds are again present as a mixture in the distillate. If two of such equally volatile compounds are immiscible in the liquid phase (heteroazeotrope), they can be readily separated from each other. However, compounds miscible with each other (homoazeotrope) can be separated in the distillate only with great difficulties according to the state of the art.
The separation of homoazeotropic mixtures made up of a C.sub.6 -C.sub.14 paraffin or paraffins and a C.sub.4 -C.sub.8 alcohol or alcohols is of great interest for various industrial processes, such as, for example, for the processing of alkanesulfonic acids.
Alkanesulfonic acids are produced, inter alia, by the sulfoxidation of paraffins in the presence of water and sulfur dioxide under the effect of light. After the reaction, a large portion of the unreacted paraffins separates in the product discharged from the reactor as a paraffin phase and can be directly recycled into the reactor.
After exhausting residues of sulfur dioxide and oxygen in the gaseous phase, the remaining product contains alkanesulfonic acids, water, paraffins, and sulfuric acid (by-product).
Several methods have been proposed for the processing of this reaction product. It is known from DOS (German Unexamined Laid-Open Application) No. 2,139,477 (page 7), which application corresponds to British Pat. No. 1,358,095, to utilize an oxygen-containing, polar organic extractant immiscible with the hydrocarbons, such as methanol, ethanol, or a low-molecular weight ester. By this polar auxiliary medium, unmixing of the mixture is effected into a paraffin phase and a paraffin-free, aqueous organic phase. The aqueous organic phase contains the alkanesulfonic acids and the sulfuric acid. After separation of the residual paraffins, again as a separate phase, the sulfuric acid is then separated as the sulfate from the aqueous phase by neutralization with an alkaline hydroxide. The presently obtained alkanesulfonates are isolated as a melt by distillation.
According to the process of DOS No. 2,139,477, a weakly polar alcohol of at least 5 carbon atoms is used as the auxiliary medium. In this case, the sulfonic acids and the paraffin are transferred into the alcohol phase, and the aqueous phase contains the entire sulfuric acid and can be separated as such. The expense of neutralizing the sulfuric acid and filtration of the sulfate according to the method disclosed in DOS No. 2,139,477 (page 7) is thus avoided. With respect to separation of the paraffins and recovery of the alcohol, the possibility is also pointed out (page 7) that the alcohol and, in some cases, the paraffins can be distilled off azeotropically with the aid of a third component; however, the further separation of such azeotropes into individual compounds is not disclosed. Moreover, in the later DOS No. 2,745,691 (corresponding to British Pat. No. 1,588,363) by the same inventors and the same assignee, precisely this circumstance is even cited as a disadvantage of the process, since--so it is stated--difficulties are encountered in using the disclosed alcohols because they exhibit a similar volatility as the feed paraffins. Furthermore, in case of homoazeotropic mixtures, separation has been possible, as discussed above, only with the aid of expensive methods, such as extraction or multistage distillation under differing pressures.
The objective thus evolving is to find a process making it possible to separate homoazeotropic mixtures made up of a mixture of a C.sub.6 -C.sub.13 paraffin or paraffins and a C.sub.4 -C.sub.8 alcohol or alcohols.